Marine diesel engines are generators of mechanical energy, but also are generators of toxic compounds into the atmosphere. The composition of the exhaust gas may be a carrier of diagnostic information about the condition of functional systems of the engine. Results of the classification and selection of diagnostic signals for selected marine engine malfunctions are presented. The analysis was based on results of laboratory tests. Mentioned classification was able to isolate symptoms of malfunctions of marine 4-stroke diesel engine in the composition of the exhaust gas. Complementary detection signals are exhaust gas temperature behind each cylinder. The conclusion of this work is the ability to detect by this method such engine malfunctions as the throttling the air intake duct, the throttling of the exhaust gas duct, the decreasing and the increasing of fuel injection pressure on the selected cylinder, chocked or discalibrated fuel injector, the leakage of the fuel pump, changing of the fuel injection timing and exhaust and inlet valves malfunctions.